Effect of wheat canopy architecture and rain characteristics on on septoria splash-borne pcynidiospore
Résumé
Canopy architecture and distances between organs of plants susceptible to fungal airborne diseases are two factors influencing disease epidemics. It has been shown that wheat architecture can affect the progression of septoria tritici blotch (STB) within the canopy. During the epidemic stage, STB progression is mainly due to rain splash, which induces upward vertical, and lateral transport of spores. We investigated, in controlled conditions, the effects of both plant architecture and rainfall characteristics on one dispersal cycle of conidia of Mycosphaerella graminicola, which are spores coming from the asexual reproduction of the pathogen fungus responsible of STB. The experiment was performed using a simulator of artificial rainfall on wheat micro-canopies (1 m2), grown in a regulated temperature greenhouse. To avoid the influence of different varietal resistance genes on disease progression, two quasi-isogenic wheat lines of the cultivar Mercia, differing only by the RHT dwarf gene, were chosen (from the John Innes Centre). A linear inoculum source of conidia suspension (106 conidia per mL), located at the centre of the canopy was set up. Each canopy was exposed to two different rainfall events: a classical stratiform-like rain with distribution of raindrop diameters centred on 2.5 mm, and a (heavy) thunderstorm-like rain with distribution of raindrop diameters centred on 3.0 mm. Just before each rainfall event, some architecture components (internode lengths, wheat heights and leaf sizes) were assessed. During each rainfall event, splash droplets were collected at different locations within the canopy to quantify spore fluxes. Thereafter, canopies were incubated in a mist chamber for 4 days, and kept 3 weeks in a greenhouse at about 20°C during the symptomless phase (corresponding to the latency period). Disease progression resulting from rain splashes in the different leaves of the plants was precisely assessed, by measurements of diseased and non-green leaf areas. We observed a strong effect of the canopy structure on the spore dispersal. A 43% smaller (and denser) canopy had a higher level of disease severity: 10 and 7 times higher for stratiform-like and thunderstorm-like rains, respectively. The smallest canopy often concentrated disease symptom on a given foliar level and the percentages of non-green foliar areas were 2 times larger than for the other canopy. Such effects were amplified with thunderstorm-like rain. Our experiment allowed quantifying the effects of the canopy architecture and the kind of rainfall event in spores dispersal and identifying interaction between them.
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